PARTICLE-SCALE INVESTIGATION OF SEEPAGE INDUCED GEOTECHNICAL INSTABILITY

土木工程经常会遇到地下水流。例子包括堤坝、防洪墙和蓄水池、地下水位下的挖掘、隧道和深地下室。在考虑其设计时，工程师们试图避免渗水所施加的浮力足以将土壤中的有效应力降低到零，从而导致隆起破坏或“流沙”的情况。通过将土壤视为连续但多孔的材料来确定这一关键情景。然而，土壤是由不同大小和形状的单个颗粒组成的。人们越来越认识到，施加在颗粒上的渗透力可以优先侵蚀桑迪中的较小颗粒。根据工程实践中使用的经典连续体计算，在被认为是安全的情况下，土壤可能会发生严重的内部侵蚀;这种现象称为内部不稳定性。我们将通过研究所涉及的基本粒子尺度机制，提高对内部不稳定性的理解，从而了解如何安全地设计和评估基础设施。国际上有几个研究小组正在对这个问题进行比较大的实验。在拟议的研究中强调粒子尺度将补充，而不是补充，这些研究。具体的研究方向来源于我们之前的研究，最近发表的其他小组的实验数据，并考虑最近公布的设计准则（如《国际堤防手册》）以及对欧洲规范EC7中岩土工程设计水力破坏准则的拟议修改。该跨机构提案将结合联合收割机在测试谢菲尔德大学（UoS）透明土壤方面的实验专业知识。在伦敦帝国理工学院（IC）学习离散元建模（DEM）。我们将以最先进的形式使用这些技术。在UoS，将开发配备激光光源的测试设施，以使土壤样品内的颗粒运动可视化，同时还使用示踪颗粒观察流体流动。三轴应力路径装置的发展，其中的围压和偏应力可以控制，同时进行这些观察将是一个特别新颖的方面，这项研究。IC将继续支持使用高性能计算，使地质力学DEM模拟和该项目将利用最近的工作是在机械工程系进行，使DEM模拟与流体流动的计算流体动力学（CFD）建模相结合。UoS和IC一直在独立工作，以审查内部不稳定的问题，因此，这一建议标志着及时的合作，以统一其互补的技能组合。例如，DEM模型可以提供有关颗粒应力的信息，而这些信息在实验室中是无法测量的，而在物理试验中可以直接观察到真实的材料的不稳定性，而无需任何数值模型中固有的任何理想化和假设。研究将澄清：（一）哪些材料最初容易受到体积变化的内部不稳定性的影响，以及最初以恒定体积（即颗粒结构的沉降或坍塌）侵蚀的材料过渡到体积变化的条件。(ii)无论是渗透速度还是水力压力梯度，都与侵蚀的发生有较好的相关性。(iii)应力水平如何影响磁化率，特别是考虑应力各向异性和主应力方向与渗流方向的关系。

Civil engineering works often encounter water flowing through the ground. Examples include embankment dams, flood walls and embankments, excavations beneath the water table, tunnels and deep basements. When considering their design, engineers seek to avoid cases where the buoyancy forces exerted by the seeping water are sufficient to reduce the effective stress in the soil to zero, resulting in heave failure or "quicksand". This critical scenario is identified by considering the soil to be a continuous, but porous material. However soil is made up of individual particles of varying size and shape. Awareness is growing that seepage forces imparted on the particles can preferentially erode the smaller particles in sandy soils. There can be significant internal erosion of the soil under scenarios that are considered safe according to the classical continuum calculations used in engineering practice; this phenomenon is called internal instability. We will improve understanding of internal instability and thereby our knowledge of how to design and assess infrastructure safely, by studying the fundamental, particle scale mechanisms involved. Internationally, several research groups are undertaking relatively large experiments of this problem. The particle-scale emphasis in the proposed research will complement, rather than supplement, these studies. The specific research direction originates from our prior research, recently published experimental data from other groups, and consideration of recently published design guidelines (e.g. the International Levee Handbook) and the proposed modifications to the hydraulic failure guidelines in the Eurocode EC7 for geotechnical design.This cross-institutional proposal will combine experimental expertise in testing transparent soil at the University of Sheffield (UoS) with skills in discrete element modelling (DEM) at Imperial College London (IC). We will use these techniques in their most advanced form. At UoS testing facilities equipped with a laser light source will be developed to enable visualization of particle movement inside soil samples while also using tracer particles to observe fluid flow. The development of a triaxial stress path apparatus where the confining and deviatoric stress can be controlled while making these observations will be a particularly novel aspect of this research. IC will continue to champion the use of high performance computing to enable geomechanics DEM simulations and the project will exploit recent work that was carried out in the Department of Mechanical Engineering to enable DEM simulations to be coupled with computational fluid dynamics (CFD) modelling of the fluid flow. Both UoS and IC have been working independently to examine the problem of internal instability and so this proposal marks a timely collaboration to unify their complementary skill sets. For example the DEM model can provide information about particle stresses that cannot be measured in the laboratory, while instability can be directly observed for real materials in the physical tests without any of the idealizations and assumptions which are inherent in any numerical model. The research will clarify:(i) Which materials are initially susceptible to internal instability with volume change and the conditions whereby a material that initially erodes at a constant volume (i.e. settlement or collapse of the particle structure), transitions to having volume change. (ii) Whether seepage velocity or hydraulic pressure gradient correlates better with the initiation of erosion.(iii) How the stress level influences susceptibility; particularly considering stress anisotropy and the relation between principal stress orientation and seepage direction.